I use the UI Builder for everything visual. Working with the XML directly I can't see the changes in real time. Notepad runs the whole time though, I use it to write out any of the scripts I need and then paste it into the UI Builder.
Plus, without the UI Builder I couldn't make the giant ugly bright purple squares I use to get the proper placements and settings of my ingame windows!
Edit:
Just for you all to laugh at me, when I first started I had the UIBuilder set to Run at all times. I thought it was impossible to work with until I read over here to turn off Run. Suddenly it became a great tool. There was a solid week of me complaining to my friends about it before I got educated. Took a crowbar to get that foot out of my mouth.
